We have developed a bioassay which measures Thy 1.2 antigen induction on null lymphocytes. It is possible to inhibit induction via beta adrenergic receptors with high concentrations of ribiquitin which also increases the sensitivity to other weak forms of induction such as with small amounts of thymopoietin and human plasma. There is a highly significant correlation between the percent of cells induced to express Thy 1.2 and the amount of thymopoietin used for induction (rho equals more than .99). This permits the expression of induction with human plasma as being equal to that amount of thymopoietin which causes a comparable level of induction. Normal plasma from children in the first age decade has the equivalent induction (mean plus or minus SD) of 13.4 plus or minus 1.5 mg thymopoietin. While it drops off by the sixth age decade it remains high with myasthenia gravis. It disappears after thymectomy. Plasma thymic hormone activity is very low or absent with the DeGeorge syndrome or with severe combined immunodeficiency. With the latter, defect levels rose after successful grafting with bone marrow fetal liver, thymus or erythrocytes. Levels are also also low with the Wiskott Aldrich syndrome and ataxia telangiectasia.